The purpose of the study proposed herein is to determine the biological function of NGF and the NGF Receptors in muscle development. The preliminary data indicate a novel role for NGF and its two receptors (p75NTR and trk A) during myoblast proliferation and differentiation. The p75NTR, a non-catalytic receptor, is expressed by proliferating myoblasts but ceases to be expressed when myoblasts fuse to form myotubes. At some point during fusion, the other NGF receptor, trk A (a tyrosine-specific kinase receptor), begins to be expressed by the fused myotubes. The expression patterns of the NGF receptors raise the possibility that NGF plays a role in myogenesis. The working hypothesis is that NGF mediates processes that mediate the viability and organization of differentiating myoblasts and the normal homeostasis of differentiated myofibers. The results of the study will hopefully reveal effects of NGF on muscle that can be exploited for therapies for preventing or treating the muscular degeneration that accompanies neurodegenerative diseases as well as Duchene's muscular dystrophy. To determine the biological function of NGF and its receptors during myogenesis, the change of expression of the receptors will be determined at different stages of myoblast differentiation in vitro. Once cell cultures have been established that mimic receptor expression in vivo, the signaling pathways the pathways activated by the p75NTR in myoblasts will be studied. Dominant negative mutations in two of the signal transduction molecules will be used to determine pathway interactions. The spatial/temporal expression of the the trk A receptor protein will be studied in vivo in order to determine the development time the receptor becomes active. To better characterize the NGF response in myocytes, C2C12 cells will be transfected with constructs that will make it possible to induce express the receptors at inappropriate times and the resulting cell lines will be tested for defects in development and differentiation. Finally, the muscle of null mutant mice for the two receptors will be examined in vivo and in vitro for changes in myoblast differentiation and muscle function. Taken together, the aims of the proposal will reveal new information about the role of NGF in the normal development and function of muscle.